Intermolecular activation of C-X (X = H, O, F) bonds by a Ti≡C tBu linkage

Article abstract of DOI:10.1021/ja0684646

The transient titanium alkylidyne complex (PNP)Ti_≡C^tBu (PNP = N[2-P(CHMe₂)₂-4-methylphenyl]₂^-) can readily activate, in some cases regioselectively, the aromatic C-H bond of anisole and fluoro-substituted anisoles to generate titanium alkylidene complexes having a substituted aryl group. Intermolecular C-O bond activation occurs when perfluoroanisole reacts with (PNP)Ti_≡C^tBu to afford the disubstituted alkylidene methoxide (PNP)Ti=C[^tBu(C₆F₅)](OCH₃). Likewise, intermediate (PNP)Ti_≡C^tBu can also promote the intermolecular C-F activation of C₆F₆ and CF₃C₆F₅ to generate disubstituted alkylidene fluorides (PNP)=Ti[^tBu(Ar_F)](F) (Ar_F = C₆F₅, C₆F₄CF₃). In the case of Ar = C₆F₄CF₃, the product resulting from para-aryl C-F activation was isolated. For the latter two C-F bond activation reactions, mixtures of alkylidene rotamers exist in solution. Complexes resulting from C-H, C-O, and C-F (including the rotamers for the latter reaction) activation have been characterized by single-crystal X-ray diffraction methods. Copyright

Full text of DOI:10.1021/ja0684646

Published on Web 04/07/2007
Intermolecular Activation of C-X (X ) H, O, F) Bonds by a TitCtBu Linkage
Brad C. Bailey, John C. Huffman, and Daniel J. Mindiola*
Department of Chemistry and the Molecular Structure Center, Indiana UniVersity, Bloomington, Indiana 47405
Received November 26, 2006; E-mail: mindiola@indiana.edu
Scheme 1. Intermolecular C-H Activation Reactions of Arenes
High oxidation state transition metal imide complexes are a well-
known class of systems which can undergo 1,2-CH bond addition
of arenes and alkanes across the MdN multiple bond.1,2 Likewise,
early transition metal alkylidenes3,4 and alkylidynes5 are also
becoming popular reagents for intermolecular 1,2-CH bond addition
of both aromatic hydrocarbons and Si(CH3)4. Despite these func-
tionalities being versatile for the hydrocarbon substrate in question,
1,2-addition reactions across MdX linkages (X ) NR or CHR)
have been limited almost exclusively to the prototypical carbon-
hydrogen bonds and, in some cases, to the weaker NH/OH bonds.6
To date, the only example of a hetero-1,2-CX bond addition (X )
F) reaction was reported by Bergman and co-workers using NC5F5.2
For the latter reaction, they attribute the C-F bond breaking step
to be driven by a combination of the inherent reactivity of the
pyridine ortho C-F bond as well as formation of a strong Zr-F
bond.2
Scheme 2. Intermolecular C-O and C-F Activation Reactions
In this work, we report that the transient titanium alkylidyne,
(PNP)TitCtBu (PNP- ) N[2-P(CHMe2)2-4-methylphenyl]2),5 can
engage in not only regioselective intermolecular 1,2-addition
reactions of aromatic C-H bonds of anisoles but also an unprec-
edented intermolecular C-O bond cleavage of CH3OC6F5, as well
as the C-F bond splitting of fluorocarbons such as C6F6 and
CF3C6F5. Consequently, the intermolecular C-O and C-F activa-
tion reactions result in formation of novel, disubstituted alkylidene
functionalities on titanium and represent the first examples of
intermolecular aryl C-O and C-F bond activation advocated by
a metal-carbon multiply bonded linkage (in addition to C-H
activation).
We recently reported that the transient alkylidyne (PNP)TitC-
tBu (A) can readily activate the C-H bonds of C6H6 or Si(CH3)4.5
Not surprisingly, when the synthon to A, (PNP)TidCHtBu(CH2-
tBu) (1), is dissolved in neat anisole, ortho-aryl C-H bond activa-
tion rapidly takes place to quantitatively afford the alkylidene aryl
(PNP)TidCHtBu(C6H4-2-OCH3) (2) (Scheme 1).7 Complex 2 likely
forms via the hypothetical intermediate (PNP)TitCtBu(CH3OC6H5),
which renders the ortho-hydrogens more susceptible to C-H bond
cleavage. Legzdins and co-workers have investigated similar C-H
bond activation reactions of anisoles with transient W neopentyl-
idenes and attribute this selectivity to be more thermodynamic rather
than kinetic in nature.4 Regioselective ortho C-H activation, with
respect to the aryl fluoride group, occurs when 1 is treated with
2-fluoroanisole to afford (PNP)TidCHtBu(C6H3-2-F-3-OCH3) (3).
Only traces of the regioisomer having C-H bond activation adjacent
to the OMe group is observed (<3% by 31P NMR spectroscopy).7
When the two ortho-hydrogens in anisole are replaced with
fluorines (2,6-difluoroanisole), bond breaking ensues at the more
vulnerable meta position to form (PNP)TidCHtBu(C6F2H2OCH3)
(4) (Scheme 1).7 Therefore, C-H bond activation occurs only ortho
to dative groups, which in this case happens to be the fluoride
substituent. Interestingly, when 4-fluoroanisole is treated with 1,
the regioisomers (PNP)TidCHtBu(C6H3-2-F-5-OCH3) (5) and
(PNP)TidCHtBu(C6H3-5-F-2-OCH3) (6) (Scheme 1) are observed
spectroscopically in a 3:1 mixture,7 respectively. This result implies
that intermediate A is somewhat discriminative for ortho C-H
activation next to the fluoride group.8 Intuitively, the ether group
is expected to be a better donor; however, preference for the ortho-
fluoride C-H bond might be the result of steric constraints imposed
by the more protected ether linkage. However, incipient negative
charge, inflicted by the fluoride, at the ortho-carbon might be
playing a crucial role in these types of reactions. Reactions leading
to formation of complexes 2-6 must be performed in neat reagent
given the promiscuity of 1 (or intermediate A) to react with any
1
solvent media.5 Complexes 2-6 have been characterized by H,
13C, 31P, and in some certain cases 19F (3-6) NMR spectroscopy.
Regioselectivity, in certain cases, was further scrutinized by single-
crystal X-ray diffraction (XRD) studies (compounds 2 and 4).7
Aryl C-H bonds are not the only linkages amenable to 1,2-
addition reactions. Hence, C-O bond cleavage occurs when 1 is
treated with CH3OC6F5, concurrent with formation of the disub-
stituted alkylidene complex (PNP)TidC[tBu(C6F5)](OCH3) (7) in
59% yield (Scheme 2).7 Complex 7 displays NMR spectroscopic
signatures consistent with a Ti-OCH3 group (1H: 3.72 ppm), an
alkylidene functionality (13C: 310.2 ppm),9 and one isomer
(rotamer). As anticipated, the solid-state structure of 7 is consistent
with the syn isomer and shows a disubstituted alkylidene carbon
(TidC, 1.953(2) Å) and a titanium methoxide ligand (Ti-O, 1.789-
(7) Å) resulting from C-O bond cleavage by the TitC linkage.7
It is well-known that the C-F linkage in C6F6 has a much higher
enthalpic dissociation energy (154 kcal/mol) than the C-H bond
9
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C O M M U N I C A T I O N S
opening metathesis of pyridines with A,12 [2 + 2] cycloaddition
could refute our original proposed mechanism and facilitate the
formation of a strong Ti-X bond through a â-X elimination
pathway (X ) OMe, F).
In conclusion, our results present a mild process by which strong
C-X bonds can be activated by a highly polarized TitC linkage.
The ability of intermediates such as (PNP)TitCtBu to intermo-
lecularly cleave aryl C-O and C-F bonds offers an excellent
opportunity to not only construct potentially important Wittig-type
reagents but also study a novel mechanism surrounding the cleavage
of strong C-heteroatom linkages. Given the heterogeneous nature
of these reactions, we are currently exploring the mechanism behind
the formation of compounds such as 7-9 using high-level DFT
studies.
Figure 1. Molecular structures of 8-syn and 9-syn depicting thermal
ellipsoids at the 50% probability level (right). Hydrogen atoms, solvent
molecules, and isopropyl methyls on phosphorus have been omitted for
clarity. Selected metrical parameters are reported in the Supporting
Information.
Acknowledgment. We thank Indiana UniversitysBloomington,
the Dreyfus Foundation, the Sloan Foundation, and the NSF (CHE-
0348941, PECASE award to D.J.M.) for financial support of this
research.
in benzene (110 kcal/mol).10 Despite this, complex 1 transforms
(48 h, 50 °C, 78% isolated yield), in neat C6F6, to a pentafluo-
rophenyl-substituted alkylidene (PNP)TidC[tBu(C6F5)](F) (8)
(Scheme 2).7 Interestingly, the 13C NMR (323.5 and 317.0 ppm)
and 31P NMR spectroscopic data for 8 reveal two alkylidene isomers
in solution (65/35 ratio).7 XRD analysis of suitable red crystals for
8 only permitted structural elucidation of isomer 8-syn (F and
perfluoroaryl groups on the same side). The structure of 8-syn also
clearly portrays a novel system bearing both perfluorophenyl and
tBu groups on the alkylidene R-C. As anticipated for a disubstituted
alkylidene ligand, the TidC linkage is long (1.946(2) Å, Figure
1). Fortunately, we found that crystals of the syn rotamer of 8, 8-syn
(red crystals), can be physically separated (85/15 ratio of syn/anti
isomers) from the reaction mixture (red crystals and orange powder),
and thermolysis of such (3 days, 25 °C) results in conversion back
to an equilibrium mixture (65/35, vide supra) of the two rotamers
8-syn/8-anti.7 The same equilibrium mixture can also be achieved
more rapidly by heating the 85% pure 8-syn at 105 °C for 1 h.
Hence, it appears that the anti isomer originates from the syn via
an intramolecular rotation mechanism.11
When octafluorotoluene (C6F5CF3) is treated with 1, intermo-
lecular aryl C-F activation in the para position ensues to afford
(PNP)TidC[tBu(C6F4CF3)](F) (9) as the major and isolable product
(Scheme 2).7 Examination of the crude mixture by 19F NMR
spectroscopy reveals at least five C-F activation products to be
generated.7 As observed with 8, predominately two rotamers are
present in solution (13C NMR: 320.6 and 315.7 ppm).7 XRD of
one of the isomers of 9 clearly portrays a disubstituted alkylidene
system bearing both perfluorophenyl and tBu groups (TidC: 1.949-
(8) Å), as well as a terminal fluoride ligand (Ti-F: 1.829(1) Å).
The C6F4CF3 group is oriented along the same side of the titanium
fluoride ligand, thus rendering this complex the syn isomer (9-syn,
Figure 1). The structure of 9-syn reveals a TidC bond length which
is significantly longer than monosubstituted alkylidene derivatives
bearing the same ancillary ligand.5 Unlike 8, however, we were
able to obtain structural data for the anti rotamer of 9, namely,
9-anti. Accordingly, XRD of 9-anti exposes the other alkylidene
rotamer having the perfluoroaryl group oriented opposite from the
metal fluoride ligand.7 The TidC linkage is comparable to the syn
rotamer (1.951(7) Å), which is also reflected by similar ArF-C-
tBu angles (116.0(4)° for 9-syn; 114.4(4)° for 9-anti). In fact, for
structures 7, 8-syn, 9-syn, and 9-anti, the plane defined by the
disubstituted alkylidene ligand bisects the PNP framework.7
On the basis of previous studies,5 we propose that complexes
7-9 are likely formed via 1,2-CO and -CF bond addition across
the transient TitCtBu bond. However, as observed in the ring-
Supporting Information Available: Experimental preparation and
reactivity (all compounds), crystallographic data (2, 4, 7-9), and
additional discussion. This material is available free of charge via the
Internet at http://pubs.acs.org.
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